The website and mobile app for Charles Schwab & Co. went offline for more than an hour Tuesday and continued to be unavailable at times on Wednesday because of a denial-of-service attack, according to the company.

"Shortly before the stock market closed today, we experienced an exceptionally high volume of website traffic which we believe was related to a denial-of-service attack," the company said in a statement. "We deeply apologize to our valued clients for the inconvenience."

Tuesday's outage lasted about an hour and 40 minutes, but Schwab tweeted Wednesday that the attacks were continuing. "We're having intermittent access issues due to a DOS attack similar to yesterday," the company tweeted. "We are actively addressing this."

The San Francisco discount brokerage company said customers could call and place trades, "although for a brief time immediately before market close call volumes were high." Schwab customer service number is 800-435-4000.

Schwab's CEO, Walt Bettinger, apologized to customers Wednesday and warned that the attacks could continue. "Based on the history of denial-of-service attacks on other companies, we anticipate these attacks may continue against our industry — and us — for some time," Bettinger wrote. "We will continue to work with the industry and law enforcement to ensure our websites are available without interruption."

For Schwab, a pioneer in online brokerages, the outage is highly unusual and potentially costly. Customers rely on quick trade execution and could claim that the online outage kept them from making profitable trades. "What if you were trying to trade Netflix?" said Mike Vorhaus, president of Magid Advisors in New York. "I'd be worried for them if it happened again in the near future."

Denial-of-service attacks typically involve thousands of computers trying to access the same website all at once, overwhelming the site's ability to respond. Typically, these attacks are made by computers infected with malware — bits of software that enable a remote user to control the machines without the owner knowing about it. Schwab said it did not know who was responsible for the attacks on its site.

TD Ameritrade, E-Trade and Fidelity all said they had not been affected by denial-of-service attacks Tuesday or Wednesday.

In USA Today's Innovators and Icons series, Charles R. Schwab, founder and chairman of Charles Schwab, discusses the current and future financial climate in the U.S. He also opens up about teaching his kids about money and learning to splurge.
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